The decision of when to buy rests on the person who is making the purchase as it would depend on a matter of factors; Someone who is buying in for the long term hodl does not bother about short term price changes and believes that in 10+ years the value is going is to much higher.
Someone who is not buying for the long term are usually more picky about when to buy and would not want to purchase when the price is on the way down. This is much more risky and would require some technical analysis and some luck as well.
yes definitely it's certained that the prices are going to multiply in 10+ years time but it's still necessary to be picky when purchasing coins though you wouldn't be as picky as one who is purchasing for a short term.
Take for instance (I love giving instances, either fictional or non fictional, it aids for clearer understanding), you purchase 1000 coins at 100$, and then because the other person is picky, he purchases the same number of coins at 20$, he may decided to keep them longer than planned and that would mean more benefits to him.